Method of and apparatus for the multi-flash distillation of liquids

ABSTRACT

THE INVENTION RELATES TO THE MULTI-FLASH DISTILLATION O LIQUIDS WHEREIN DISTILLATION IS EFFECTED BY A PLANT COMPRISING A SERIES OF DISTILLATION STAGES EACH HAVING A HEAT EXCHANGER AND ASSOCIATED FLASH CHAMBER, A HEATER INTRODUCED BETWEEN THE FINAL HEAT EXCHANGER AND THE FIRST FLASH CHAMBER, THE HEATER AND FLASH CHAMBERS BEING ALL CONNECTED IN SERIES BY MEANS BY WHICH THE PRESSURE IS REDUCED IN SUCCESSIVE CHAMBERS.

Oct' lo' 1972 A. PoRTEoUs METHOD 0F AND APPARATUS FOR THE MULTI-FLASH DISTILLATION OF LIQUIDS f Filed NOV. 17, 1969 QU zQGwm m55 02:00u @mim 3,697,382 Patented Oct. 10, 1972 3,697,382 METHOD F AND APPARATUS FOR THE MULTI- FLASH DISTILLATION 0F LIQUIDS Andrew Porteous, Bearsden, Dunbartonshire, Scotland,

assignor to National Research Development Corporation Filed Nov. 17, 1969, Ser. No. 877,145 Claims priority, application Great Britain, Nov. 19, 1968, 5,483/ 68 Int. Cl. B01d 3/10, 3/06, 3/02 U.S. Cl. 202-173 2 Claims ABSTRACT 0F THE DISCLOSURE The invention relates to the multi-flash distillation of liquids wherein distillation is effected by a plant comprising a series of distillation stages each having a heat exchanger and associated llash chamber, a heater introduced between the final heat exchanger and the lirst Hash chamber, the heater and flash chambers being all connected in series and means by which the pressure is reduced in successive chambers.

According to the invention the residence time of the liquid passing through at least two of the flash chambers is so controlled that not more than 97% equilibration is obtained therein and in the preceding chamber the percentage of equilibration is marginally less to initiate the operating percentage of equilibration in the said succeeding chambers thus ensuring an increase in the temperature difference between the liquid and the llashed vapour in the said chambers.

The invention also consists in the plant wherein the said residence time is controlled by reducing the size of the ash chambers.

This invention has reference to the multi-Hash distillation of liquids.

A multi-Hash distillation plant comprises essentially a series of distillation stages each having a heat exchanger and an associated tlash chamber, a heater introduced -between the linal heat exchanger and the lirst llash chamber, the heat exchangers, the heater and the liash chambers being all connected in series in the order given, and means by which progressively lower pressure is maintained in successive flash chambers.

Indistillation by such plant the temperature of the liquid is raised in passing through the successive heat exchangers by the vapours of their associated ash chambers, the temperature of the liquid is further raised by the heater and the liquid then passed through the successive flash chambers in each of which a fraction of the liquid is llashed, the vapour being condensed by the respective heat exchangers.

The flashing of the liquid in each chamber is due to the saturation temperature of the vapour therein being below the temperature of the inowing liquid.

Heretofore multi-flash distillation plants have been so designed and operated that the residence time of the liquid in passing through each liash chamber is such that it flashes to obtain equilibrium of temperature with that of the vapours therein and leaves at the saturation temperature of the respective chambers. Should the liquid be a solution such temperature is increased by the boiling point elevation of the solution.

Percentage of equilibration is expressed by the for-mula:

TBi- TBD l Tiom.) X where TB, is the liquid inlet temperature -to a stage TBo is the liquid outlet temperature from the same stage t, is the vapour saturation temperature corresponding to vapour .pressure in the stage te is the boiling point elevation if the liquid is a solution.

In prior art practice the equilibration attained is greater than 99% in each stage.

The object of the present invention is primarily to provide an improved method of ilash distillation whereby for the same output as is obtained by the iiash distillation plants as heretofore constructed the size of the plant is substantially reduced.

Another object of the invent-ion is to provide a ash distillation plant which will be substantially smaller than such plants of known construction which have the same output and performance ratio which latter is defined as the pounds ot distillate produced per 1,000 B.t.u. of heat input.

According to the present invention in the distillation of liquids by the liash method carried out in at least four chambers in series the residence time of the liquid in passing through each of at least two of the flash chambers connected in series is so controlled that not more than 97% equilibration as `above dened is obtained in said two .chambers and in the preceding chamber the percentage ,of equilibration is marginally less than the percentage of equilibration in the following two or more chambers to initiate the operating percentage of equilibration in the said succeeding chambers thus ensuring an increase in the temperature difference between the liquid and the flashed vapour in the said chambers and the liquid leaving said chambers.

In preference the residence time in all but the last ashboth the method and the apparatus with reference to the accompanying drawings wherein:

FIG. l shows diagrammatically an example of a multiflash distillation plant of known construction; and

FIG. 2 shows diagrammatically a plant in accordance with the present invention and designed to give the same output for the liquid flow rate as in the plant shown in FIG. l.

Both plants have twenty-tive stages, only the flash chambers designated 1, 2, 3, 22, 23, 24 and 2S and their associated heat exchangers 1a, 2a, 3a, 22a, 23a, 24a and 25a are shown. The heater is designated 26 and is introduced between the end heat exchanger 1a and the first flash chamber 1.

I .asrrssz -v The heatexchangers 25a, 24a, 23a, 22a to 3a, 2a, 1a,

the heater 26 and the ash chambers 1, 2, 3 to 22,-23,.24 v

and 25 are all connected in series. The flows of liquid being distilled, the distillate and vapours are indicated p the intake.28 to the first heat exchanger 25a.

The pressures in the flash chambers are successively decreased from therst to the last chamber in known f manner.

It will be assumed that the liquid being distilled is seawater-brine.

Sea-water-brine passes through the heat exchangers from heat exchanger 25a to heat exchanger 24a and during its passage its temperature is raised by condensation of the vapour from the chamber 25. The vapour saturation temperatures'are shown below the heat exchangers.

In like lmanner the sea-water-brine passes through the heatexchangers of the succeeding stages and is heated by the vapours from the respective associated dashing chambers. The pre-heated sea-water-brine now passes through the heater 26. to further raise the temperature thereof finally to 204'? F. The operation of both plants up to this point is similar.

Reference will now be directed to the operation of the 4plant shown in FIG. 1.

The sea-water-brine now preeheated to 204 F. enters flash chamber 1 wherein part of the liquid is flashed, the vapour at 200 F. saturation temperature passing to the heat exchanger 1a the sea-Water-brine nowat 200 F. passes to flash chamber 2. In this chamber part of the liquid is tiashed, the vapour at 196 F. saturation temperature passing to heat the heat exchanger. 2a and the sea-.water-brine now at 196 F. passing to the next flash chamber.

In like manner the liquid is flashed at each stage, the temperature dropping 4 F. at each stage, and a fraction of the liquid is flashed to vapour and is condensed by the respective heat exchanger. In each flash chamber the residencetime is such that the percentage equilibration vis greater than 99%. That is the liquid reaches virtually the saturation temperature at the pressure existing in the respective chambers. *l Reference will now be made to FIG. 2.

The sea-water-brine entering chamber 1 has a shortened residence time so that it is flashed and passed to chamber 2 at 201 F. 'I'he residence time in the vfirst stage is such that it initiates the departure from equilibrium which is subsequently made use of up to stage 25. That is, the residence time in stage 1 is not sufficient for 99% equilibation to be obtained. The sea-water-brine at 201 F. then enters chamber 2 where its residence'time is such that its temperature is reduced to 197 F., that ishere again 99% equilibration is not attained, i.e. there .is no equilibrium of temperature between the vapour and seawaterfbrine stream in the second stage.

The liquid flows through the other flash chambers in each of--which, except the last chamber 25, `the residence time is such that its temperature does not fall to the saturation temperature of the respective chamber. In the finalV 4 and designed to operate with at least 99% equilibration to. produce the same `stage distillatev output. Although the drawings are diagrammatic the relative dimensions of the corresponding chambers in the vtwo plants are diagrammatically indicated.

'Ihe reduction of dimensions of the said flash chambers gives the following advantages:

(1) A substantial reduction in the chamber dimensions.

(2) The .use of higher sea-water-brine flow rates/unit width and hence the construction of larger capacity MS'F plants than those at present known.

(3) Y"A substantial reduction in supporting' and reinforcing steelwork.

(4) A substantial reduction in plant foundation preparation, thereby facilitating constructional matters.

The temperatures and percentagesof equilibration used are only illustrative of how the process operates.

FIGS. l and 2 are also only illustrative. The design percentage of equilibration may be greater or less. The initial unequilibration may commence in'the jth stage where j is less than the total number of Stages in the plant and is located so thatv at least two stages follow in which the equilibration is 97% vor less. Ify desirable various ranges of unequilibration maybe used in the same plant. The particular choice of parameters is left to the designer.

It is to be understood that in the invention neither-the proportion nor the actual quantity of seawater-brine which is equilibrated is kept absolutely constant from stage to stage but the proportion in all stages where the invention is used is less than or equal to 97% equilibration. The devices which control the passage of the unequilibrated sea-waterbrine according to the invention may be of any appropriate form. Thus the iiowof the unequilibrated sea-water-brine from stage to stage can be controlled by passing it through flow control devices such as sharp edged orifices 29 of appropriate dimensions and located between the stages.

It will be understood that the sizeof the heat exchangers must always be such that they will condense the quantities of vapours produced. l

What I claim is:

1. A method of distilling a liquid by the flash method in which the liquid is passed through a plurality' of flash chambers connected in series which comprises controlling the residence time of the liquid in each of at least two -of the flash chambers by the volumetric capacity thereof so that not more than 97% equilibrium as expressed by the formula: v'

is obtained in the volumetrically modified chambers-and wherein the percentage of equilibrium in the chamber rpreceding the first of said modified chambers is marginally less than the percentage of equilibriumV in said succeeding modified chambers to initiate the operating percentage of equilibrium ,in said succeeding chambers thus ensuring an increase' in the temperature difference between the liquid and the flashed vapour in `said succeeding'charnbers. v

2. A-multi-llash distillation plant comprising a series of `distillation stages` each having a heat exchanger.V and 'an vassociated ash chamber, a heater introduced betweenthe final heat exchanger and the first flash chamber, the :heat exchangers and the -flash chambers being all connected in series in the order given, the volumetric capacity of at least two of the chambers being such that not more than 97% equilibration as expressed by the formula:

TBP (tri-te) (100) where TBi is the liquid inlet temperature to a stage TBo is the liquid outlet temperature from the same stage ts is the vapour saturation temperature corresponding to vapour pressure in the stage te is the boiling point elevation if the liquid is a solution is obtained in the volumetrically modied chambers and wherein the percentage of equilibration in the chamber preceding the rst of said modified chambers is marginally less than the percentage of equilibration in said succeeding modified chambers to initiate the operating per- UNITED STATES PATENTS 3,398,059 8/1968 Cane et al. 159-2 MS 3,427,227 2/ 1969 Chamberlin 202173 3,431,179 3/1969 Starmer 203-11 X 3,533,917 10/1970 Williams 202-173 NORMAN YUDKOFF, Primary Examiner J. SOFER, Assistant Examiner U.S. Cl. X.R. 203-11, 88 

